If it's been oked by multiple electricians, it's probably OK. As your article states, properly installed knob and tube is at least as safe as modern wiring.
The major problem is that since knob and tube lost favor and today, demand for electricity within the home has grown considerably, and a lot of knob and tube systems were expanded improperly; this can happen with modern wiring as well. Also, the insulation issue.
Knob & tube left in place is generally safe. If you want to do anything at all with the electrical system, however, you'll need to upgrade the whole system.
The major problem is that since knob and tube lost favor and today, demand for electricity within the home has grown considerably, and a lot of knob and tube systems were expanded improperly; this can happen with modern wiring as well. Also, the insulation issue.